1. Field of the Invention
The present invention relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to certain aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such characteristics of a patient. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry measures various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.
Pulse oximeters typically utilize a non-invasive sensor that emits light into a patient's tissue and that photoelectrically detects the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount related to the amount of a blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of the blood constituent in the tissue using various algorithms.
The pulse oximetry measurement depends in part on the assumptions that the contribution of outside light sources is negligible and that the detected light is transmitted through relatively homogeneous tissue. However, outside light may leak into a sensor, causing detection of light that is not related to the amount of blood constituent present in the blood. Additionally, these assumptions fail to take into account that human tissue is by nature heterogeneous, and that within any given tissue site there may be variations in the size and location of large blood vessels, bones, connective tissue, and other subcutaneous anatomic structures. These structures affect the path of the light as it passes through the tissue, causing measurement variations that do not relate to amount of the blood constituent.